1. Field of the Invention
The present invention relates to a structure having pores, a magnetic recording medium, and a method of manufacturing the magnetic recording medium.
2. Description of the Related Art
Magnetic recording media represented by hard disks have been drastically progressed in recent years, and the recording density has increased at a rate as high as 100% per year. Though at a laboratory level, the recording density up to 50 Gb/in2 is achieved at present. Intensive studies and developments are continued with an expectation of a further improvement in recording density.
Presently, magnetic recording on a hard disk is performed by a longitudinal recording method, also called an in-plane recording method, in which magnetization is recorded in the horizontal direction with respect to a substrate. In the in-plane recording method, bit information is recorded and reproduced with a magnetic head by utilizing a leaked magnetic field from a magnetization transit region provided between adjacent magnetized recording areas. Each of the magnetized recording areas provided on a track has a nearly rectangular shape with a size of 300 μm (bit width) corresponding to the track width and 30 nm (bit length) corresponding to the direction of length of the track. The magnetization is oriented in the direction of length of the track. With the conventional in-plane recording method, as the bit length is reduced to improve the recording density, the leaked magnetic field from the magnetization transit area is also reduced. This results in a problem that the leaked magnetic field can no longer be detected if the bit length becomes too small. The problem can be avoided by reducing the film thickness of a magnetic layer. However, this solution raises another problem that an extreme reduction of the bit volume causes a super-paramagnetic state in which the direction of magnetization is changed with the effect of thermal energy, and hence the recorded magnetization cannot be properly retained. Taking into account those problems, it is thought that an upper limit of the recording density obtainable with the in-plane recording method is approximately 100 Gb/in2. On the other hand, in the recording density range beyond 100 Gb/in2, a vertical recording method is more potentially expected in which a magnetic substance having magnetic anisotropy in the vertical direction with respect to a substrate is employed as a recording layer and magnetization is recorded in the vertical direction with respect to the substrate.
In contrast with the in-plane recording method, the vertical recording method has a property that a demagnetizing field is reduced at a higher density. Also, because a sufficient film thickness of the magnetic layer can be maintained even with an increase of the recording density, the vertical recording method is superior to the in-plane recording method in resistance against the super-paramagnetic state caused with the effect of thermal energy. For those reasons, it is thought that the vertical recording method is more highly potential than the in-plane recording method in the recording density range beyond 100 Gb/in2. In the vertical recording method, a Co—Cr alloy is generally used as a recording layer. When the Co—Cr alloy is formed on a substrate made of, e.g., Si, glass or carbon by sputtering, Co and Cr grow in a state in which the two components are separated. More specifically, a portion containing a larger amount of Co component has a columnar shape and provides a ferromagnetic area having a hexagonal close-packed (hcp) structure, which serves as a recording area. A portion containing a larger amount of Cr component, which grows in a surrounding relation to the columnar recording area, is a nonmagnetic area and serves to weaken the magnetic interaction between the adjacent recording areas.
Further, a magnetic recording medium has also been proposed in which a structure including a magnetic material buried in a nonmagnetic material to serve as a columnar recording area, as described above, is artificially formed in a regular pattern by the lithography. One example of such a magnetic recording medium is manufactured by the steps of forming pores regularly arrayed on a substrate through a series of processes of coating a resist on a vitreous carbon substrate, patterning the resist with electron beam drawing, and etching the patterned resist, then filling a magnetic material NiFe in the pores by sputtering, and polishing the substrate surface such that the magnetic material and the nonmagnetic material have flat surfaces (see Japanese Patent Laid-Open No. 2000-277330).
In a magnetic recording medium of the type called patterned media, which is featured in recording one bit in each of the above-mentioned pores filled with the magnetic substances, unlike magnetic recording media utilizing the in-plane recording method and the vertical recording method, a structure more suitable for higher density recording is obtained because a recording area is made up of the pores filled with minute magnetic substances, and the magnetic substances are arrayed in the same shape and the same size. Such a magnetic recording medium has received big attention as a medium expectable in the next generation because it can realize the recording density of 1 Tb/in2 by further reducing the pore size and the pore pitch.
On the other hand, among general pore forming methods, a method of forming pores with anodic oxidation of Al is known as one capable of forming a large number of minute pores with a less variation in shape. With that known method, an anodic oxide film is formed as a porous anodic oxidation coating through anodic oxidation of an Al substrate performed in an acidic electrolyte, such as sulfuric acid, oxalic acid or phosphoric acid (see, e.g., R. C. Furneaux, W. R. Rigby & A. P. Davidoson & NATURE, Vol. 337, p 147 (1989)). The porous coating has a peculiar geometric structure featured in that very minute columnar pores (almina nanoholes) each having a diameter of several nanometers to several hundreds nanometers are arrayed in parallel at intervals of several tens nanometers to several hundreds nanometers. Those columnar pores have a high aspect ratio and also have high uniformity in diameters of their sections. Here, when the pore is quadrilateral, e.g., rectangular, in section, the aspect ratio is represented by y/x where x is the length of a longer side (length of one side in the case of a square shape) and y is the pore depth. When the pore is circular in section, the aspect ratio is represented by y/x where x is the pore diameter and y is the pore depth.
Also, the structure of the porous coating can be controlled to some extent by changing the conditions of anodic oxidation. For example, it is known that the pore pitch, the pore depth and the pore diameter can be controlled to some extent respectively depending on the anodic oxidation voltage, the anodic oxidation time, and the pore widening process. Here, the pore widening process means an alumina etching process, which is usually carried out as a wet etching process using phosphoric acid.
Further, to improve verticality, straightness and independence of pores in the porous coating, there is proposed a method of carrying out the anodic oxidation in two stages, i.e., a method of carrying out the anodic oxidation to form a porous coating, removing the porous coating, and then carrying out the anodic oxidation again to form a porous coating with pores having higher verticality, straightness and independence (see Japanese Journal of Applied Physics, Vol. 35, Part 2, No. 1B, pp. L126–L129, Jan. 15, 1996). This proposed method is based on that dents of the Al substrate, which are obtained after removing the anodic oxidation coating formed by the first anodic oxidation, serve as start points for pore formation in the second anodic oxidation.
In addition, to improve controllability of the shape, pitch and pattern of pores in the porous coating, there is proposed a method of forming start points for formation of the pores using a stamper, i.e., a method of depressing a board having a plurality of bosses on its surface against the surface of an Al substrate, thereby forming dents as start points for pore formation, and then carrying out the anodic oxidation to form a porous coating with pores exhibiting better controllability of the shape, pitch and pattern of the pores (see Japanese Patent Laid-Open No. 10-121292 (Nakao) and Masuda, Kotai Butsuri (Solid State Physics), 31, 493 (1996)). Also, there are known a method of forming dents as start points for pore formation by irradiating an FIB (focused ion beam) onto a substrate surface, and a method of forming dents as start points for formation of patterned pores by uniformly coating a resist resin on substrate surface, patterning the resist by the photolithography or the electron beam lithography, and then dry etching the substrate surface.
Applying alumina nanoholes to patterned media by filling magnetic substances in the alumina nanoholes formed by the anodic oxidation is a technique already known. In the case of employing the alumina nanoholes, it is also possible to obtain a medium of the in-plane recording type if the magnetizing direction of the magnetic substance filled in each of the alumina nanoholes can be controlled in the in-plane directions, and to obtain a medium of the vertical recording type if the control can be performed in the vertical direction. However, the alumina nanoholes are usually in the columnar form and the columnar pore structure is more suitable for a medium of the vertical recording type from the viewpoint of physical structure. When the alumina nanoholes are applied to a medium of the in-plane recording type, the following drawbacks may occur.
In the columnar nanoholes, an energy difference due to anisotropy in shape of the nanoholes in the in-plane directions is not caused because of a circular section. This leads to a possibility that magnetization may rotate in the plane, or a possibility that the direction of magnetization is not uniform for all of the nanoholes. If the direction of magnetization is not uniform, the relative positional relationship between magnetization of the magnetic substances filled in the nanoholes and a magnetic head for detecting the magnetization is changed for each nanohole. Consequently, precise recording and reproduction on and from the magnetic recording medium cannot be achieved.